Interstellar burp leads to discovery of new pulsar

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Astrophysicist makes her second pulsar
find

Dec. 11, 1998:
Few people get to discover a new star. Colleen Wilson-Hodge,
an astrophysicist at NASA's Marshall Space Flight Center, now
has two with her second discovery of an astrophysical oddity
- a pulsar orbiting a massive star.

Pulsars - rotating neutron stars - are among the most intriguing
objects in the sky. They were found in 1965 when radio astronomers
discovered several objects that emitted radio waves with clock-like
precision.

Left:
A sky map showing the approximate location of this discovery (J1946+274). The pulsar and its companion are located
near the familiar constellation of Cygnus. At this time of year,
Cygnus is only briefly visible to Northern observers just after
sunset. With an apparent visual magnitude of +14.4, J1946+274
is not visible to the naked eye or to small telescopes.

The sources were identified as rapidly rotating neutron stars
with intense magnetic fields. Another type of pulsar, called
an accretion powered X-ray pulsar, was first detected in 1971
by an X-ray satellite called Uhuru. Whereas radio pulsars have
the regularity of a Swiss watch, accretion pulsars are like cheap
alarm clocks that often gain and lose time - and go off when
you least expect it.

The massive companion, a type B[e] star, is a superhot
blue-white star (type B) about 8 to 15 times as massive as our
sun, with a distinctive signature - emission lines caused by
glowing hydrogen and oxygen blown off the star.

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Pulsars are rotating neutron
stars with strong magnetic fields (about 1011
to 1012 Gauss, similar to magnetars, but
with weaker magnetic fields), formed when a large star goes supernova
and compresses its core as it blows off its outer layers, or
when a white dwarf accretes enough material to force gravitational
collapse. The pulsating nature comes from the rapid rotation
of the neutron star whose magnetic field axis is not aligned
with its spin axis. As material is funneled onto the magnetic
poles of the star, energy from this accreted matter is released
in the form of X-rays in the cone-shaped beams depicted above.
The concentration of emitted X-rays from the magnetic poles appears
to a distant observer as a periodic pulse of radiation as the
beam sweeps past the observer, much like the rotating beam of
a lighthouse.

Wilson-Hodge discovered her first pulsar in 1995 with the
Burst and Transient Source Experiment (BATSE) aboard the Compton
Gamma Ray Observatory, and studied it with additional observations
by the Rossi X-ray Timing Explorer (RXTE). Besides being the
12th known transient accreting X-ray pulsar with no visible companion,
it went off twice each orbit, rather than once, because its lopsided
orbit took it through the B[e] star's excretion disk twice
each orbit. GRO J2058+42, as it is called, appeared to be orbiting
a B[e] star.

Wilson-Hodge's second pulsar discovery came on Sept. 7, 1998.
At the same time, astronomers operating the RXTE's All-Sky Monitor
noticed the same burst pattern in their data.

"They got a better location and we got a better period,"
said Wilson-Hodge, illustrating the value of having more than
one set of eyes studying the sky.

An earlier set of eyes - Canada's Ariel satellite - may have
seen the pulsar in 1976. The position for 3A 1942+274 lies within
the RXTE and BATSE error boxes, but there's enough uncertainty
that no one can be sure.

Above: A simplified diagram shows the pulsar's inclined
orbit through the B[e] star's circumstellar disk. As the
pulsar passes through the and absorbs matter from the excretion
disk, created when the B[e] star ejects matter, the pulsar
bursts in brightness. (Note that this diagram does not show the
distortion in the disk that would be caused by the massive gravitational
field of the pulsar.)

This pulsar has two complex names, XTE J1946+274, and GRO
J1944+26, acknowledging its dual discoverers. The numbers give
the position in the sky - right ascension (celestial longitude
eastward from the vernal equinox: 19h 46m) and declination (angle
above or below the equator: +27.4 degrees) - for the center points
of the Rossi XTE and BATSE estimates of the position. Other observations
have refined the position to 19h 45m 34s+27deg 30.0 min, and
provided a strong hint that the visible companion indeed is a
type B[e] star about 13,000 light years away.

Wilson-Hodge found a 15.8-second period in the data, clearly
making the object a pulsar. Nothing rotates that fast unless
it is very compact, and that has to be a neutron star or black
hole (for various reasons, it's not the latter).

"About half of these sources are known to orbit type
B[e] stars. We think the rest are similar. Visible companions
haven't been found for all of them because some are too far away
and others are hidden by dust between us and the star.""Several
of these pulsars that we've seen with BATSE have started out
with a giant outburst and have followed with a series of regularly
spaced smaller outbursts," she continued. Scientists suspect
the giant outburst is caused by a massive ejection of material
from the B[e] star into an excretion disk surrounding
the B[e] star.

The pulsar then orbits through it and causes the big outburst.
The smaller outbursts occur as the pulsar orbits through again
and again, gradually sweeping the area clean and possibly dragging
some material in its trail.

"What causes these ejections by the B[e] star
is not known," she said. "These B[e] stars seem
to be rotating very rapidly, but not quite at breakup speed,"
she noted. As they spin, they cast out large quantities of gas
in an expanding disk which the neutron star passes through during
its orbit.

"We get a giant outburst," she said, "from
matter being dumped onto the surface of the neutron star."

With the giant outburst having announced the presence of the
star, Wilson-Hodge and other scientists now are awaiting the
"normal" bursts to see if they can determine the orbital
period and other secrets of this oddity. The giant outburst is
still going on, but it has faded to about 25% as bright as its
peak of 35 milliCrabs (3.5% the brightness of the Crab Nebula).